'''qmail''' is the second most popular mail transport agent, behind '''sendmail'''. It was written in reaction to sendmail.  Its main design goal was to be secure, and in its seven years of existence, has never had a security flaw. One of the keys to meeting this goal is by being made up of about twenty-five simple C programs. The biggest one is 1600 lines long, the others are 200-600.

If you know anything at all about sendmail, qmail is an awesome system. The code is crystal clear (well, if you have programmed in C for a decade) and the overall system is relatively easy to understand. Its author, DanBernstein, is a genius programmer and qmail is a masterpiece.

Like much open source software, qmail is light on design documentation. It's worth getting the big picture first, then reading the man page before jumping into the code.

* http://www.nrg4u.com/ - the "big qmail picture"
* http://www.qmail.org/man/index.html - man page
* http://cr.yp.to/qmail/guarantee.html - DanBernstein describes his design goals
* http://cr.yp.to/qmail.html - home page
* http://www.qmail.org/ - mirror sites

I think that qmail is an ideal system for people to study who want to be good programmers. If you have any criticisms of it, please let me know. This page is a good place.

-- RalphJohnson